This invention relates generally to electrochemical machining (ECM), and more specifically, to methods and systems for forming cooling holes in a turbine engine airfoil.
Electrochemical machining (ECM) and/or shaped tube electrochemical machining (STEM) is commonly used to form cooling holes in turbine engine airfoils. During an ECM process, the workpiece being machined is coupled to a positive terminal of a DC power supply and the electrode is coupled to a negative terminal of the DC power supply. An electrolyte flows between the electrode and the workpiece. For example, the electrolyte may be an acid or an aqueous salt solution. During the machining process, the workpiece is dissolved by controlled electrochemical reactions to form the cooling hole. Generally, such machining processes form cooling holes that have a circular cross-sectional area. The circular cross-sectional area is sized to provide a desired amount of flow metering through an inlet of the cooling hole, however, the circular cross-sectional area often reduces an amount of potential heat transfer inside the blade and/or downstream of the cooling hole, thereby reducing an amount of cooling within the blade. Further, a circular cross-sectional area defined at an outlet of the cooling hole generally may not be suitable for an airfoil having a narrow trailing edge. Moreover, a circular cross-sectional area defined at an outlet of the cooling hole may induce high stress concentrations in an area of the airfoil surrounding the cooling hole.
Accordingly, at least some known cooling holes are designed with an inlet having a circular cross-sectional area and an outlet having an elliptical cross-sectional area. Generally such cooling holes are formed via an electrochemical machining process. Although such cooling holes have the benefits of both a circular cross-sectional area and an elliptical cross-sectional area, such cooling holes also include an expansion area defined between the circular and the elliptical cross-sectional areas. As such, airflow within the expansion may experience an abrupt flow change and become disrupted and substantially non-uniform. Moreover, fabrication of such cooling holes is generally more expensive and time consuming than fabrication of other cooling holes, as known machining methods require at least two passes of an electrode to form the cooling hole.